Treatment with hydrocortisone succinate significantly reduced the magnitude of the plaque-forming cell (PFC) response to the helper T cell dependent form of Type III pneumococcal polysaccharide (SSS-III), but had no effect on the response to SS-III, with or without treatment with antilymphocyte serum. Also, this drug had no influence upon the induction or expression of suppressor T cell activity. These findings imply that helper T cells are steroid sensitive, whereas those T cells (amplifier and suppressor cells) which regulate the magnitude of the antibody response to SSS-III are steroid resistant.